1. Field of the Invention
The present invention relates to a system and a method of detecting chipping produced on the boundary of an object of detection and, more particularly, to a chipping detection system and method suitable for detecting chipping in the visual inspection of a slider of a thin film magnetic head portion of a disk driver.
2. Description of the Related Art
In a slider of a thin film magnetic head, the rail part comes into contact with a disk. For this reason, if there is chipping, there is a fear of damaging the disk, so that very strict quality control must be carried out.
The chipping detection is conventionally carried out with human eyes, but in place of such detection various techniques for automatically detecting chipping has been proposed. Some of the conventional techniques relating to this kind of system are disclosed, for example, in Japanese Patent Laid-Open No. 255484/1986 and Japanese Patent Publication No. 13617/1987. In the former method, a straight line is applied by a least square method to the boundary of the straight line portion of a binary image obtained by the detection through a TV camera or the like and the values of the binary image are examined along the straight line. From the results, the presence or absence of a chipping is judged. In the latter method, scattered light produced by chipping is detected, and for this purpose, the incident angle and the disposition of a scattered light receptor are optimized.
These two conventional techniques respectively have the following problems.
In the former technique, since approximation using a least square method is necessary, calculation requires a long time or the scale of hardware is enlarged, thereby making it impossible to obtain a high-speed and small-scale system. In addition, since a chipping at the corner portion of an object of detection is not taken into consideration, it is impossible to detect a chipping at a corner portion.
In the latter technique, since the positional relationship between the scattered light receptor and an object of detection is fixed, it is necessary to position the object of detection accurately, thereby requiring additional large-scale equipment.
The positional relationship between the projector and the scattered light receptor is not always common to a plurality of configurations, so that it is impossible to deal with plural kinds of objects by one apparatus.
For these reasons, a chipping detection system which enables high-speed inspection by a small-scale apparatus and chipping detection at the corner portion, which is capable of easily positioning an object of detection, and which is capable of inspection of plural kinds of objects by one apparatus has been demanded.
The contour of the rail surface is conventionally bevelled by what is called chamfering so as to protect the contour against chipping.
If it is assumed that the chamfering angle for bevelling the contour of the rail surface so as to have an inclined surface is .alpha., when illuminating light enters the chamfered portion, it is reflected regularly in an oblique direction at an angle of 2.alpha., so that only part of the light returns to an object lens.
Consequently, the chamfered portion is also dark in the detection like a chipped portion. Particularly, at the corner portion of the rail surface, since chamfering of the front surface is overlapped with chamfering of the side surface, the chamfering angle is apt to become large, thereby exerting great influence on the detection.
Therefore, since the corner portion becomes round on the binary image and the portion appears as if it had a defect, false information which judges the normal part as a defect is sometimes produced.